Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. Transmission speed ratio is the ratio of input shaft speed to output shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Generally, transmissions include at least one negative speed ratio which is engaged when the driver selects reverse.
The driver controls the transmission mode by manipulating a range selector. Range selectors typically provide PARK, REVERSE, NEUTRAL, and DRIVE modes. In PARK mode, the vehicle is held stationary even in the presence of substantial force. This is conventionally implemented by placing the transmission gearbox in a neutral state and holding the output shaft stationary with a parking pawl. In REVERSE, the vehicle moves backward when the driver presses the accelerator pedal. In NEUTRAL, pressing the accelerator pedal does not cause the vehicle to move although the vehicle is allowed to move in response to other forces such as gravity. Finally, in DRIVE, the vehicle moves forward when the driver presses the accelerator pedal. REVERSE, NEUTRAL, and DRIVE are conventionally implemented by placing the gearbox in a corresponding state with the parking pawl released.
The transmission has a number of gear sets with gear teeth that are engaged when the vehicle operator selects a particular shift position. Typically, there is a gear lash, or play, between the gear teeth. As such, in some cases, when the operator shifts from the DRIVE shift position to the REVERSE shift position or the REVERSE shift position to the DRIVE shift position, the meshing gear teeth, due to the gear lash, may cause noise, vibration, and harshness (NVH) concerns as the gear teeth strike each other. Ultimately, this may result in an objectionable “clunking” sound and tactile disturbances.